There is an ever-increasing demand for energy in all parts of the world. One of the problems encountered in the production of energy is the resultant production of pollutants. One of the major pollutants is sulfur which is present in varying concentrations in both coal and oil.
There have been many attempts to remove sulfur from oil via hydrorefining and similar techniques. These techniques work well, as long as the oil does not contain too high a content of metal and particulates which complicate hydrorefining. Efforts to desulfurize coal have not to date been proven on a commercial scale. The preferred way to remove sulfur pollutants from some heavy oils, and most coals, is to burn the fuel and to remove the sulfur oxides from the flue gas.
Many processes are known for removal of sulfur oxides from flue gases. One of the most interesting methods proposed commercially for the removal of sulfur oxides from flue gases is the Shell Flue Gas Desulfurization (SFGD) process. In this process, a dry acceptor material is used to capture sulfur oxides from flue gas. The acceptor material is periodically regenerated with a regeneration gas, usually a gas containing 50% or more of H.sub.2 and relatively concentrated sulfur oxides are recovered in the regeneration off gas. The advantage of this process is that the acceptor material is dry so the complications and disposal problems associated with wet scrubbing systems are avoided. Some SFGD reactor designs provide for parallel passage of flue gas along side of the surface of the acceptor material. This avoids plugging of the acceptor with particulates.
The net effect of the SFGD process is to take very dilute sulfur oxides, primarily SO.sub.2, in an oxygen and particulate containing stream, and to produce a relatively more concentrated stream of SO.sub.2 gas in an oxygen-free atmosphere. Typical SO.sub.2 concentrations in flue gases may range from only a few ppm up to 1 volume percent. These gases are concentrated and recovered in the regeneration of the acceptor materials to produce a regeneration gas containing about 2 to 10 volume percent of SO.sub.2. When using the preferred regeneration material, hydrogen, the concentration of SO.sub.2 may be up to about 10% with very pure hydrogen. Hydrogen such as produced in refineries and containing only about 50 mole percent hydrogen may be used, and in these instances, the regeneration off gas will contain only about 2 to 5 mole percent SO.sub.2. This SO.sub.2 is still rather dilute and not generally an acceptable feed to a Claus unit which converts SO.sub.2 into Sulfur. To improve the purity of the SO.sub.2, and also to minimize the swings in SO.sub.2 concentration caused by the cyclic regeneration procedure used on the acceptor material, an absorber-stripper is used to produce concentrated SO.sub.2.
Unfortunately, the cyclic nature of the regeneration is still felt in the down-stream absorber-stripper used to purify the regeneration off gas.
As the absorber-stripper is a very necessary part of an SFGD unit wherein very high concentrations of SO.sub.2 are required, and because the stripping column in the absorber-stripper requires a significant portion of the total utilities demand of the SFGD process, the inefficient operation of the absorber-stripper caused by cyclic swings in concentration of regeneration off gas has been a significant problem.
The SFGD process is, of course, well known in the art and forms no part of my invention. A general discussion of the SFGD process is given in an article entitled "New Tool Combats SO.sub.2 Emissions", The Oil and Gas Journal, Oct. 29, 1973, page 81. The problem caused by cyclic operation of the absorber-stripper has also been given further study by the developers of that process. In U.S. Pat. No. 3,764,665 (Class 423/574, 244, 539, 576 and 55/73), the teachings of which are incorporated by reference, a significant improvement in the operation of the absorber-stripper was disclosed. Basically, the improvement was providing a large buffer zone between the absorber and the stripper. The function of the buffer zone was to be a large holding tank which would dampen out the swings in concentration of rich liquid from the absorber. Thus, the stripper would see a stream of almost constant composition. Another alternative disclosed was use of a smaller buffer zone with variation in flow of rich liquid from the buffer zone to the stripper based on the concentration of SO.sub.2 in the rich liquid. Flow of rich liquid to the stripper would be adjusted to maintain constant the production of SO.sub.2 in the off gas from the stripper.
The improvement suggested in this patent, although a significant advance at the time, still was not a complete solution to the problem. Use of a very large buffer zone or holding tank meant that a very large vessel would be required to provide any significant improvement in operation. As the customary time of regeneration of the acceptor material is 30 to 100 minutes, the attempt to provide a buffer zone is analogous to providing a fly wheel on a one cylinder engine which operates at one revolution every 30 minutes. A fly wheel will be able to dampen out power pulses, but it will have to be an exceedingly large fly wheel. Another disadvantage of using a large buffer zone is that it forces the stripper to operate on the average concentration of H.sub.2 S. Thus, the easiest stripping occurs with the richest solutions, those containing the most SO.sub.2. The function of the stripper is to concentrate SO.sub.2 in the overhead vapor and to provide an SO.sub.2 free stream. When very little SO.sub.2 is in the gas being charged to the absorber, very little SO.sub.2 will be in the rich liquid from the absorber, the stream is still lean. Mixing of streams rich and lean in SO.sub.2 results in a significant loss of entropy in the system. Similarly, when there is very much SO.sub.2 in the gas being charged to the absorber, the rich liquid from the absorber will have a very high concentration of SO.sub.2. Mixing of this concentrated stream with the relatively dilute stream in the buffer zone also increases the entropy of the system.
The alternative teaching in this patent, namely, varying the flow to the stripper to make sure of a constant production of SO.sub.2, also has its failings. In this mode of operation, flow to the absorber will be lowest when concentration of SO.sub.2 in the absorbing liquid is highest. Similarly, when not much SO.sub.2 is in the regeneration off gas, very high flows will be required to maintain a constant production of SO.sub.2 in the stripper off gas. This also results in high flows of stripped liquid back to the absorber, at a time when the requirement of stripped liquid in the absorber is lowest because of the low concentration of SO.sub.2 in the gas. Another problem caused by varying flow rates is that the mechanical design of the absorber is more complicated because of the greater variation in flow rates which must be tolerated.
I have now discovered a way to permit operation of the absorber-stripper wherein the production of SO.sub.2 gas from the stripper is constant, and the liquid flow rates through both the absorber and the stripper are also constant. Further, the utilities required to strip SO.sub.2 from the rich liquid are significantly less than disclosed in any other prior art schemes. Specifically, I am able to operate the absorber-stripper according to the method of the present invention at a utilities cost of only about one-half that of the rest of the prior art processes.